We had an hour at a Montessori preschool when my dd1 was four. Her friend had toured it the day before and really liked it and since my friend (his mom) and I wanted to keep the kids together, we went to spend some time there too - to see if we would like it also.
Little background - our kids have known each other since they were tiny babies. Spent a lot of time together as they were growing up. My dd is very social; friend's ds is not so much. They attended co-op preschool together when they were three, as well as many other things.
So df arrives before we do. When we got there it just felt odd to me but I am used to co-op so that is my adjustment needed. Df is "working" when my dd goes up to him, says hi, asks if she can play with him. He says sure. As she is reaching for something on his tray (they choose a tray to "work" on during the time period we were there), the teacher actually RUNS from across the room to swoop in and get between df's tray and my dd. Tells her she can use it after df is done and replaces it on the shelf. This plays out a number of times until my dd is in tears as she is not allowed to play WITH anyone. I can clearly see that this is not the school for us but before we go, dd tells me (VERY loudly - and this school was oddly quiet for the number of young children there) "THIS IS NOT MY SCHOOL! This is a bad school. That is a bad teacher!" I am just trying to shuttle her out as soon as possible but it took a minute to put our shoes back on (not allowed to wear shoes there), get coats, etc.
In their defense (well, not in defense of that specific school - I really cannot defend them...) but in defense of Montessori in general - they do have cool toys and do teach independence which is of course important. They had items that taught real life stuff - like a small pitcher of water and little cups to practice pouring. That was cool and my dd would have loved it had she been allowed to explore it with a friend.
I feel the extreme down side (IMO of course) is that for the social child - and even for the ones who are not, maybe even more important for them? - it didn't allow for the socialization/problem solving that I think is such a big reason to go to preschool in the first place. I also found Montessori to be too rigid for my taste - there is a certain way to do things and they must be done in that way. I don't think it is very respectful of creativity but I have a very limited knowledge of it so I'm sure Montessori fans will correct me.
It really depends on your child's personality. My friend's ds went to "the bad school" (my dd STILL remembers and calls it that, lamenting that her friend went there) for two or three years and loved it. It wasn't the right place for my kid but there are tons of Montessori preschools around and they seem to be doing just fine so I'm sure they must work great for plenty of kids/families. You asked for opinions re: social kids and I sure do have one of those!